(a) Field of the Invention
The present invention relates to an objective for video disks and, more particularly, to an objective for reading out the signals recorded on high-density information-recording disks (video disks).
(b) Description of the Prior Art
For objectives to be used in reproducing systems for video disks, it is required to warrant resolving power of 1.mu. due to the fact that the objective has to read out very small signals recorded with high density. Moreover, the information read out from the disk, which rotates at high speed, contains signals for making the objective follow up the recorded track and signals for automatic focusing in addition to image information. To make the objective read out those information and signals correctly, the flatness of image focused by the objective should be high. To prevent breakage of the video disk and objective which will be caused when the objective contacts the video disk, the working distance of the objective should be long. Besides, to perform automatic focusing, the objective should be compact and light in weight. Moreover, the price of the objective should be low.
As the light used for the objective for video disks is generally a monochromatic light (.lambda.=6328 A), it is effective for eliminating the noise at the time of amplifying the signals from a detector when transparency for the light of this wavelength is as high as possible. Therefore, to make transparency high, it is necessary to provide multi-layer anti-reflection coating on the lens surface or to make the number of lenses constituting the objective as small as possible. When this problem is considered in connection with the above-mentioned other requirements such as low price and light weight, it is more advantageous when the number of lenses constituting the objective is made as small as possible.
Some of known objectives for video disks are arranged to meet the above-mentioned requirements. However, known objectives for video disks have N.A. about 0.4 to 0.45 and comprise about four lenses. When, however, semi-conductor laser is used as the light source, the wavelength of light is long and, therefore, an objective with large N.A. is required. Moreover, for an objective for video disks with high recording density, large N.A. is required. But, known objectives for video disks do not meet this requirement satisfactorily. Therefore, it is required to provide an objective for which aberrations are favourably corrected up to large N.A. If, however, the number of lenses is increased in order to obtain large N.A., it is disadvantageous for obtaining a light-weight objective.